Hydroxy aromatics are valuable organic intermediates in chemical industries related to resin, plastics, pharmaceuticals and agrochemicals. Phenol for example is further processed to form phenol resins, caprolactam, bisphenol A, adipic acid. Similarly cresols are used for preparing antioxidants, herbicides, insecticides, dyes, flavoring agents, plastics and lubricating oils. p-Cresols is also used for producing BHT (2,6-di-tert-butyl-4-hydroxy toluene) an important antioxidant. Guaiacol (o-methoxyphenol) and p-methoxyphenol also find wide applications in the field of pharmaceuticals, synthetic perfumes, antioxidants and polymerization inhibitors.
There are a number of processes for preparing hydroxyaromatics. Phenol is conventionally produced from benzene via cumene process which involves alkylation of benzene with propylene to yield cumene, oxidation of cumene to cumene hydroperoxide by air and its cleavage in acidic medium to equimolar amounts of phenol and acetone. This process has several limitations such as multistep synthesis, generation of acetone as inevitable side product and lower yields of phenol. Cresol is produced from toluene by multistep reaction involving sulphonation, chlorination or by vapour phase methylation of phenol. In the first process toluene is sulphonated with concentrated sulphuric acid and the sodium salt of sulphonated toluene is fused with sodium hydroxide at 300° C. to yield a mixture of o-, m-, and p-cresols. The p-cresol is separated from the mixture by traditional crystallization. The process has disadvantages that large amount of sodium sulphite is produced as byproduct. Second process involves chlorination of toluene with sulphur dichloride in the presence of iron chloride, separation of chlorotoluene isomers followed by hydrolysis and distillation to yield pure o-, m-, and p-cresols. The process has the disadvantages like low yields of p-cresol, production of byproducts such as tolyl cresols and tolyl ethers. Cresol synthesis by vapor phase methylation of phenol yields only o-cresols and 2,6-xylenol, uses relatively expensive raw material phenol and require high capital cost corrosion resistant plant.
There are mainly two industrial processes for the production of guaiacol involving methylation of catechol with dimethyl sulphate or carbonate in presence of alkali. p-Methoxyphenol is industrially produced by selective methylation of hydroquinone with dimethyl sulphate or carbonate. These processes are highly environmentally unacceptable.
The one step process for direct hydroxylation of aromatic hydrocarbons has therefore attracted word wide attention and several methods for direct hydroxylation of benzene to phenol, toluene to cresols and anisole to guaiacol, p-methoxyphenol have been reported in the literature. There are many references in the prior art for direct hydroxylation of aromatics to hydroxyl aromatics and the important ones are discussed here.
A process using ZSM-5 as catalyst and N2O as oxidant has been reported for one step hydroxylation of benzene to phenol (J. Mol. Catal. 1993, 84, 117; Appl. Catal. A: Gen. 1992, 86, 139; Appl. Catal. A: Gen 1994, 117, 1). The main limitations for industrial application of this process are high operating temperature (>200° C.), deactivation of catalyst due to coke formation, limited availability and high cost of N2O oxidant. Another process using divalent iron as catalyst and hydrogen peroxide as oxidant has been reported for direct hydroxylation of benzene to phenol (J. Chem. Soc. 1969, 2897). Another process for direct hydroxylation of benzene to phenol describes the oxidation of benzene with H2 and O2 using a palladium catalyst supported on TS-1 (J. Chem. Soc. Chem. Comm. 1992, 1446). Yet another process describes the use of molecular oxygen as oxidant and poly (metal) salt of dihyroxyanthraquinone dissolved in water as catalyst for direct hydroxylation of benzene to phenol (U.S. Pat. No. 4,982,600). Still another process describes the synthesis of phenol by catalytic one step oxidation of benzene using TS-1 as catalyst and hydrogen peroxide (prepared insitu by reaction of O2, CO and water in presence of palladium complex as catalyst) as oxidant (U.S. Pat. No. 5,981,764). Still another process described the use of ammonium salt of mono vanadium (V) substituted heteropoly amines as catalysts for direct hydroxylation of benzene to phenol with hydrogen peroxide (J. Mol. Catal. A: Chem. 1991, 67, 7). Yet another process for Liquid phase direct hydroxylation of benzene to phenol using titanium and vanadium containing zeolites as catalysts and hydrogen peroxide as oxidant have been reported (J. Mol. Catal. 1992; Micro. Mater. 1994, 2, 451). Another literature report (J. Mol. Catal. 1997, 126, 43) describes a process for one step oxidation of benzene to phenol wherein, a mesoporous molecular sieve, MCM-41 either exchanged with copper ions or loaded with copper oxide was used as the catalyst, 10 atmospheric pressure of oxygen as oxidant, ascorbic acid as reducing agent and acetic acid as solvent. A recent, U.S. Pat. No. 6,180,836 B discloses a process for direct liquid phase hydroxylation of benzene to phenol, which involves the use of hydrogen peroxide as oxidant and molecular sieves doped with copper ion as catalyst. Another recent report (J. Mol. Catal. 2000, 156, 143), describes direct hydroxylation of benzene to phenol by using hydrogen peroxide as oxidant and vanadium (V) substituted polyoxomolybdates as catalysts. Yet another literature report (J. Mol. Catal. 2006, 253, 1) describes sodium metavanadate catalyzed direct hydroxylation of benzene to phenol using hydrogen peroxide as oxidant and acetonitrile as solvent. Still another literature report (Appl. Clay Sci. 2006, 33, 1) describes the selective hydroxylation of benzene to phenol using hydrogen peroxide as oxidant and clay supported vanadium oxide as catalyst. Yet another literature report (Catal. Today 1999, 49, 285) describes direct hydroxylation of benzene to phenol, toluene to cresols and anisole to methoxyphenols under solvent free triphasic conditions using TS-1 as catalyst and H2O2 as oxidant.
A process for direct hydroxylation of toluene to cresols by using N2O as oxidant and pentasil or β-type zeolites as catalysts with conversion rate 24% and selectivity 24% have been reported in DE A-196,34,406. Another process for direct hydroxylation of toluene to cresols by using dinitrogen monooxide (N2O) as oxidant, iron-containing zeolite as catalyst in the temperature range 275 to 450° C. has been reported (U.S. Pat. No. 5,110,995) and conversion rate 48% with selectivity for cresol 20% have been achieved. Another process for direct hydroxylation of toluene to cresols by using zeolites which have been passed through a special two stage calcinations process as catalyst, N2O as oxidant with conversion rate 25% and selectivity for cresol 22% has been reported (EPA 889, 081). Yet another patent (U.S. Pat. No. 6,476,277 B2) describes a process for direct hydroxylation of toluene to cresols by using nanocrysatline zeolites as catalysts and N2O as oxidant.
A process for direct hydroxylation of anisole to methoxyphenols with H2O2 in CF3SO3H containing small amount of H3PO4 has been described in U.S. Pat. Nos. 4,223,165 and 4,301,307. Another process where phenolic ethers and phenol derivatives have been directly hydroxylated with hydrogen peroxide in the presence of acidic clays has been described in EP 314, 583. Yet another process for direct hydroxylation of anisole to methoxyphenols with H2O2 in presence of titanosilicate catalyst and cyclic ethers like THF, dioxane as solvent has been described in U.S. Pat. No 5,426,244.
The drawbacks of the hitherto known processes such as lower activity of the catalyst, less conversion, poor selectivity, evident the necessity for development of an improved process for direct hydroxylation of aromatic hydrocarbons like benzene, toluene and anisole.